The method of electrophotographic development is a method in which toner particles in a developer are made to adhere on an electrostatic latent image formed on a photoreceptor to develop the image. The developer used in this method is classified into a two-component developer composed of toner particles and ferrite carrier particles, and a one-component developer using toner particles alone.
As a development method using a two-component developer composed of toner particles and ferrite carrier particles among these developers, a cascade method and the like were formerly employed, but a magnetic brush method using a magnet roll is now in the mainstream.
In a two-component developer, ferrite carrier particles are a carrier substance which is stirred with toner particles in a development box filled with the developer to thereby impart a desired charge to the toner particles, and further transports the charged toner particles to a surface of a photoreceptor to thereby form a toner image on the photoreceptor. The ferrite carrier particles remaining on a development roll holding a magnet are again returned from the development roll to the development box, mixed and stirred with fresh toner particles, and used repeatedly in a definite period.
In a two-component developer, unlike a one-component developer, ferrite carrier particles have functions of being mixed and stirred with toner particles to charge the toner particles and transporting the toner particles, and have good controllability on designing a developer. Therefore, the two-component developer is suitable for full-color development apparatuses requiring a high image quality, high-speed printing apparatuses requiring reliability and durability in image maintenance, and other apparatuses.
In a two-component developer thus used, it is needed that image characteristics, such as image density, fogging, white spots, gradation and resolving power, exhibit predetermined values from the initial stage, and additionally these characteristics do not vary and are stably maintained during endurance printing. In order to stably maintain these characteristics, properties of ferrite carrier particles contained in the two-component developer need to be stable.
Patent Literature 1 (Japanese Patent Laid-Open No. 2012-230373) describes a resin carrier made by using, as a magnetic carrier core material, spherical composite particles composed at least of ferromagnetic iron oxide microparticles and a cured phenol resin, and coating the surface of the particles with a resin.
Patent Literature 2 (Japanese Patent Laid-Open No. 2012-58344) describes a ferrite carrier core material composed of porous ferrite particles, whose composition is represented by (MnO)x(MgO)y(Fe2O3)z, wherein part of (MnO), (MgO) and/or (Fe2O3) is substituted with SrO in 0.3 to 4.0% by weight, and a resin-filled ferrite carrier for an electrophotographic developer prepared by filling the pores with a resin.
Patent Literature 3 (Japanese Patent Laid-Open No. 2007-34249) describes obtaining an electrophotographic developer carrier by adding resin particles, a binder, a dispersant, a humectant and further water to a raw material powder, wet grinding and drying the mixture to thereby obtain a granulated product, calcining and thereafter sintering the granulated product to thereby manufacture a carrier core material having a hollow structure in its interior, and coating the carrier core material with a resin.
In recent years, the employment of a trickle development system has increased the amount of a carrier used, whereas in order to suppress the running cost of a developer, attempts to reduce the amounts of the developer and a carrier used have been made.
As a method of reducing the amount of a carrier used, there is proposed a method of using a carrier having a low apparent density. In the case of using a developing machine having the same volume, when the apparent density is low, the weight of a carrier used is enabled to be reduced.
On the other hand, when a carrier coated with a resin is intended to be used over a longer period, the coating resin exfoliates due to stresses in the interior of a developing machine to resultantly vary the charging property and the resistivity to form images and often make it unable for stable images to be provided over a long period.
Although the low-temperature fixation of toners is being progressed from the viewpoint of the energy saving, the progress of the low-temperature fixation makes the toners to be liable to be adhered to the surface of carriers due to stresses in the interior of a developing machine to resultantly vary the charging property of the carriers and raise apprehension of adversely affecting images.
Although resin-filled carriers and resin carriers are studied in order to solve these problems, there arises such a problem which is that the carriers are liable to have a high resistivity and the control of the resistivity of the carriers is difficult because the carriers use a large amount of a resin. Further since the charging property also is largely influenced by the resin, the use condition as a carrier is likely to be limited.
That is, in order to solve these problems, a carrier is demanded not only which has a low specific gravity but also whose charging property and resistivity can easily be controlled.